Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

MEMBRANE PROTEINS
581
lipid
bilayer
Figure 10–22 Steps in the folding of a
multipass transmembrane protein.
When a newly synthesized transmembrane
α helix is released into the lipid bilayer, it is
initially surrounded by lipid molecules. As
the protein folds, contacts between the
helices displace some of the lipid molecules
surrounding the helices.
newly synthesized multipass
transmembrane protein
folded
membrane protein
multipass membrane protein structures to be determined by x-ray crystallography.
The number of β strands in a β barrel varies widely, from as few as 8 strands
to as many as 22 (Figure 10–23).
β-barrel proteins are abundant in the outer membranes of bacteria, mitochondria,
and chloroplasts. Some are pore-forming proteins, which create water-filled
MBoC6 m10.25/10.24
channels that allow selected small hydrophilic molecules to cross the membrane.
The porins are well-studied examples (example 3 in Figure 10–23C). Many porin
barrels are formed from a 16-strand, antiparallel β sheet rolled up into a cylindrical
structure. Polar amino acid side chains line the aqueous channel on the inside,
while nonpolar side chains project from the outside of the barrel to interact with
the hydrophobic core of the lipid bilayer. Loops of the polypeptide chain often
protrude into the lumen of the channel, narrowing it so that only certain solutes
can pass. Some porins are therefore highly selective: maltoporin, for example,
preferentially allows maltose and maltose oligomers to cross the outer membrane
of E. coli.
The FepA protein is a more complex example of a β barrel transport protein
(Figure 10–23D). It transports iron ions across the bacterial outer membrane. It
is constructed from 22 β strands, and a large globular domain completely fills the
inside of the barrel. Iron ions bind to this domain, which by an unknown mechanism
moves or changes its conformation to transfer the iron across the membrane.
Not all β-barrel proteins are transport proteins. Some form smaller barrels that
are completely filled by amino acid side chains that project into the center of the
barrel. These proteins function as receptors or enzymes (Figure 10–23A and B);
the barrel serves as a rigid anchor, which holds the protein in the membrane and
orients the cytosolic loops that form binding sites for specific intracellular molecules.
Most multipass membrane proteins in eukaryotic cells and in the bacterial
plasma membrane are constructed from transmembrane α helices. The helices
Figure 10–23 β barrels formed from
different numbers of β strands.
(A) The E. coli OmpA protein serves as a
receptor for a bacterial virus. (B) The E. coli
OMPLA protein is an enzyme (a lipase) that
hydrolyzes lipid molecules. The amino acids
that catalyze the enzymatic reaction (shown
in red) protrude from the outside surface of
the barrel. (C) A porin from the bacterium
Rhodobacter capsulatus forms a waterfilled
pore across the outer membrane.
The diameter of the channel is restricted
by loops (shown in blue) that protrude into
the channel. (D) The E. coli FepA protein
transports iron ions. The inside of the barrel
is completely filled by a globular protein
domain (shown in blue) that contains an
iron-binding site (not shown).
lipid bilayer
EXTRA-
CELLULAR
SPACE
2 nm
(A) 8-stranded
OmpA
(B) 12-stranded
OMPLA
(C) 16-stranded
porin
(D)
22-stranded
FepA
PERIPLASM